Because adding all possible ranks would be to much. We just chose one of the Lieutenants at random. We just need one, because the only reason for a Lieutenant to be there is to replace the Captain in case he is incapable of leading the company. Even if you wanted to break into platoons, you would still have a leader for each platoon. More sergeants or officers would just bloat up the company structure for no reason.

From the blog:

Quote

You might have noticed that the company is not having the correct composition of ranks that a company should have had in reality. This has three reasons. Firstly, it was not uncommon during the civil war, that a company would be reduced so much in size that many ranks were simply missing or no longer needed. Secondly, it would blow the scale of our game if we were to include every single rank that a regiment/company would have had.